wmaker-check: rewrote parsing of structure fields for callback checker

wmaker-check: rewrote parsing of structure fields for callback checker

Doug Torrance reported a problem with some versions of AWK which caused the
signed/unsigned attribute to not be properly ignored, because the regular
expression was using a GNU extension to keep it simple.

While trying to reproduce it, I discovered that mawk was chocking on one
regexp used in parsing the type+names parsing of structure members.

This patch is rewriting the parsing, because hacking regexp to make them
fall working is still not the best solution for maintainability. There is
now a clearer code which is also safer because it will handle more
gracefully corner cases.

Reported-by: Doug Torrance <dtorrance@monmouthcollege.edu>
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>